1. Technical Field
This invention relates to cordless telephones and more particularly to a cordless telephone having a plurality of portable units arranged for communicating with a base unit in a frequency hopping system.
2. Description of the Prior Art
The Federal Communications Commission (FCC) recently promulgated rulings in the utilization of spread spectrum systems, including a frequency hopping system. These rulings now allow suppliers to produce improved cordless telephones which provide users much greater freedom and mobility than is available with conventional cordless telephones. Frequency hopping systems spread their energy by changing, or hopping the center frequency of the transmission many times a second in accordance with a pseudo-randomly generated list of communication channels. The result is a significantly higher signal to noise ratio than may be achieved by conventional techniques such as amplitude modulation that uses no bandwidth spreading.
These improved cordless telephones provide security naturally from an eavesdropper listening in on a conversation simply because of the spread spectrum/frequency hopping transmission technique employed by these telephones. Examples of such improved cordless telephones are described in U.S. Pat. No. 5,323,447 which issued to M. E. Gillis et al. on Jun. 21, 1994, U.S. Pat. No. 5,353,341 which issued to M. E. Gillis et al. on Oct. 4, 1994 and U.S. Pat. No. 5,463,659 which issued to W. J. Nealon et al. on Oct. 31, 1995. Although these improved cordless telephones give users much more freedom and mobility than conventional cordless telephones, they do not permit simultaneous, multiple access for the cordless telephone portable units to an air interface. As a result, the multiple cordless telephone portable units are unable to share or join in a call link in progress with a respectively associated base unit. Moreover, a cordless telephone base which has two telephone lines is unable to communicate with more than one of the portable units at any given time. Also, the number of portable units that can be configured to communicate with the respectively associated base unit is limited to a finite number.